1. Field of the Invention
The present invention is related to an LED lighting device, and more particularly, to an LED lighting device with high power factor and adjustable color rendering index.
2. Description of the Prior Art
An LED lighting device directly driven by a rectified alternative-current (AC) voltage usually adopts a plurality of LEDs coupled in series in order to provide required luminance. As the number of the LEDs increases, a higher forward-bias voltage is required for turning on the LED lighting device, thereby reducing the effective operational voltage range of the LED lighting device. As the number of the LEDs decreases, the large driving current when the rectified voltage is at its maximum level may impact the reliability of the LEDs. Therefore, there is a need for an LED lighting device capable of improving the effective operational voltage range and the reliability.
Additionally, color rendering refers to the effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant, as defined by the International Commission on Illumination (CIE). the color rendering index (CRI) is a quantitative measure of the ability of a light source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. Generally speaking, light sources with a high CRI are desirable in color-critical applications such as photography and cinematography. However, the CRI of a conventional LED lighting device, generally 60-70, is very poor. Therefore, there is a need for an LED lighting device capable of improving the effective operational voltage range, the reliability and the color rendering index.